Self-healing reinforced concrete structures and process for the preparation thereof



Sept. 16, 1969 o. R. HULL L 3,466,822 1 SELF-HEALING REINFORCED CONCRETE STRUCTURES AND PROCESS FOR THE PREPARATION THEREOF Filed April 28, 1967 FIG.1

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INVENTORS v JOHN AUGUSTUS PICCARD United States Patent 0.

U.S. Cl. 52-223 3 Claims ABSTRACT OF THE DISCLOSURE Structures of concrete or other hardenable cements are strengthened and rendered self-healing to a substantial extent by reinforcement with imbedded cross-laid groups of pretensioned synthetic yarns.

BACKGROUND OF THE INVENTION Concrete structures have been strengthened by glass filaments as shown by Jackson U.S. Patent 2,425,883 even by prestressed rods of resin-bonded glass fibers as in Goldfein U.S. Patent 2,921,463. Prestressed strands have been embedded in resin compositions as seen in Rubenstein U.S. Patent 2,850,890.

SUMMARY OF THE INVENTION This invention relates to prestressed reinforced structures of concrete or other hardenable inorganic cements, and more particularly to such structures in the form of thin sheets that are reinforced with cross-laid groups of pretensioned yarns of small diameter molecularly oriented continuous filaments formed from synthetic organic polymers.

Neither glass filaments nor high tensile strength steel wire are suitable for use as reinforcement for the thin concrete or cement structure contemplated herein. Glass fiber or steel wire reinforced thin cement structures lack one or more of the qualities possessed by the structures of the present invention. Thus, the former may lack in adhesion between the matrix and the reinforcement or they may not exhibit self-healing qualities when the structure is distorted and the forces acting to distort the structure are subsequently removed.

Thin concrete panels reinforced with staple-length fibers of asbestos are relatively brittle and have been completely shattered when subjected to an impact force, which, under similar conditions inflicted only relatively minor damage to the present reinforced panels without substantially changing their original dimensions or contour.

In contrast to pretensioned reinforcing media of the prior art, most drawn filaments of synthetic polymers exhibit relatively high degrees of work and tensile recovery at elongations greater than 5%. Once the reinforced structure is distorted beyond the elastic limit of the concrete and cracks are formed, additional energy of impact is absorbed in recoverable elongation of the filaments, since the amount of energy required is less than would be necessary to overcome the adhesion at the fiberc-oncrete interface. Upon removal or dissipation of the distorting force, the elastic property of the filaments acts to restore the structure to its original form. If only a minor amount of spalling (i.e., loss of concrete) has occurred, the compressive force exerted by the reinforcing network will be sufiicient to close the cracks to the point where they are no longer visible to the casual observer. When compared with steel wires, multifilament yarns or cords of synthetic organic polymers exhibit a high degree of adhesion to concrete. This is believed due, at least in 3,466,822 Patented Sept. 16, 1969 part, to the greater surface area and surface contours of the fibrous reinforcing agents.

BRIEF DESCRIPTION OF THE DRAWING FIGURES 1 and 2 illustrate the top and cross-sectional side views respectively of a preferred apparatus for fabricating the reinforced panels of this invention. FIGURE 3 is a cross-sectional view of the reinforced panel.

The structural sheets of the invention are prepared using a pretensioning apparatus such as that which is schematically represented in FIGURES l and 2 in the accompanying drawings. One end of each yarn 1 of the reinforcing network is secured to an anchor pin 2, mounted on the main frame 3 of the apparatus. The remaining end of the yarn is secured to an anchor pin that is mounted on a movable plate 4 which is, in turn, operably engaged to a suitable means (not shown) for moving the plate toward and away from the main frame in the direction indicated by the double-headed arrows.

Pretensioning of the filamentary group is achieved by increasing the distance between the main frame and movable plates. Four slotted guides 5 are employed to maintain an equal distance between each of the filamentary members during pouring and distribution of the aqueous concrete slurry. The required amount of concrete mix is poured on to the reinforcing network. A bottom plate 6 is located beneath the network and is approximately equal in area to the network. A suitable vibrator 7 beneath the bottom plate is activated, allowing the concrete to be evenly distributed over the reinforcing network. The top plate 8 is then placed in position and urged downward until it rests on all four height adjustment screws 9. The top plate is preferably equipped with a second vibrator 10. The intensity of the vibration is adjusted to achieve maximum fluidity of the concrete without causing cavitation. This permits the entrapped air bubbles to rise to the surface and be broken during spreading of the concrete.

When the concrete has hardened sufliciently to withstand the compressive force exerted by the pretensioned reinforcing network, the filamentary members are cut at their point of emergence from the concrete. To facilitate removal of the hardened product, all flat surfaces that contact the concrete are covered with polyester film. The film is preferably secured to the plate beyond the perimeter of the contact area. In FIGURE 3, yarn ends 11 represent the lower group and yarn 12 a part of the upper cross-laid group embedded in concrete 13 of the panel.

DETAILED DESCRIPTION The present invention provides reinforced thin structures, i.e. 1 inch or less of cement or concrete which, when distorted, exhibit high levels of form recovery and crackclosure, together with a preferred process for fabricating these structures.

The panel or thin structure of the invention comprises a matrix of portland cement or other hardenable inorganic cement with a pretensioned grid-like network formed by two or more groups of substantially parallel yarns of continuous synthetic filaments laid crosswise, imbedded in and completely surrounded by the matrix. The network combines high tensile strength, elastic recovery and stress retention over a wide range of elongation with good adhesion to the cement matrix. The groups are generally parallel to and coextensive in dimensions with the major surfaces of the reinforced sheet. The filaments of the yarns are elongated by at least 5% of their relaxed length and are maintained under stress. Adjacent yarns within a warp are spaced no more than 81 apart, where t represents the thickness of the prestressed reinforced sheet. The angle of intersection between yarns in adjacent warps is from 45 to Within the volume enclosed by 3 the grid-like network, the volume ratio of fiber to matrix should not exceed 1:1.

The novel process for fabricating the prestressed reinforced concrete panel comprises the following steps:

(1) Laying crosswise two or more groups of yarns or cords of molecularly oriented continuous filaments of a high tenacity linear synthetic organic polymer.

(2) Elongating the filaments by at least about of their relaxed length.

(3) Placing an aqueous mix of concrete or other inorganic cement so as to completely surround the cross laid warps. If desired a dry mix may be applied and water added subsequently.

(4) Allowing the concrete or cement to cure while the yarns are under tension. By this is meant that tension on the yarn must be maintained at least to the point where the cement will withstand the compressive force exerted by the pretensioned groups upon release of tensioning.

A preferred product of this invention comprises a thin panel of portland cement containing a sand aggregate. The panel has a thickness of at least 0.13 inches (0.33 cm) and midway through the thickness of the panel and parallel with respect to the major surfaces thereof is a grid-like network comprising two or three cross-laid groups of substantially parallel yarns of drawn and prestressed high tenacity molecularly oriented filament of polyhexamethylene adipamide, The yarns of adjacent groups appear to contact one another at substantially all crossing points, the angle of intersection being about 90. In this one particular embodiment the yarns are in the form of a twisted cord formed from 2-140 continuous filament yarns of 6 denier per filament.

Adjacent yarns or cords in any group are equally spaced about 0.13 inches (0.33 cm.) apart. During prestressing the cords are elongated by as much as 15% of their relaxed length and are under a long term stress of greater than about 3 grams/ denier. The total compressive force acting on the panel is about 400 p.s.i. (2800 grams/ sq. cm.). The character of the yarn surface prevents slippage along the yam-concrete interface, which would result in loss of prestress.

Within the zone bounded by the groups, the volume ratio of fiber to matrix should not exceed about 1:1 in order to avoid delamination due to insufficient bonding between matrix layers on either side of the reinforcing network.

In order to achieve the required compressive force on the sheet without the delamination which can result from insufficient spacing between adjacent yarns, the yarns must exhibit a tenacity and elongation-at-break of at least 5 grams per denier and 9% respectively. These measurements are made at 70 F. (21 C.) and 65% relative humidity using an elongation rate of 10% per minute.

To maintain the sheet under substantial compressive stress for an appreciable length of time, the yarn should also exhibit the following characteristics:

(1) Good adhesion to the matrix.

(2) Resistance to degradation by the uncured alkaline concerte mixture.

(3) A rate of stress decay that does not exceed 40% of the initial value over a period of 4000 minutes at ambient temperature (70 F The stress decay rate is determined as follows: The yarn to be tested is secured to the clamps of a tensile tester such that the unstretched, slack-free length of the sample is 10 inches (25.4 cm.) prior to application of stress. The sample is then stretched at a rate of 1 inch (2.5 m.) per minute to achieve either an increase in length of or a length that is 1% less than the elongation at break, whichever is less. One minute after the yarn is strecthed to the desired length, the total stress on the yarn is recorded and the reading is referred to as initial stress. The yarn is left undisturbed in the tester in the stretched state for 4000 minutes. The stress value after this period of time is measured and labelled final stress.

4 The difference between the two stress values should not exceed 40% of the initial value.

A preferred process for fabricating the reinforced concrete product of the present invention comprises arranging two or more cross-laid warps of polyamide yarns or cords on a suitable pretensioning apparatus and elongating the filaments of the yarns by between 10 and 15% of their relaxed length. The settable concrete mix is applied so as to embed the cross-laid groups, the overall thickness preferably not exceeding one half inch, The viscosity of the concrete mix is generally such that it remains in the desired area. If not, a container may be employed. Preferably, the apparatus is vibrated during and following pouring of a concrete slurry to evenly distribute the concrete through and around the area of the cross-laid groups. The filaments comprising the network are severed at their point of emergence from the concrete once the concrete has hardened sufficiently to withstand the compressive stress exerted by the reinforcing network upon release of tensioning. The location of the reinforcing network in the concrete matrix will ordinarily be determined by the application for which the structure is designed. For example, in the case of residential siding, the reinforcing network is best located as far as possible from the exposed surface.

The following examples demonstrate the superior form recovery and crack-closing ability that distinguish the reinforced sheet structures of the present invention from similar structures reinforced with metal Wires. While the examples serve to illustrate a preferred embodiment, they should not be interpreted as limiting the scope of the present invention. Unless otherwise specified, the samples illustrated in the following exam les are square panels that are 12 inches (30 cm.) along each side.

The following tests were employed to demonstrate the superior strength and crack-closing ability exhibited by the reinforced panels of the present invention.

Falling ball impact test A 4 x 4 inch (10 x 10 cm.) section was cut from the panel and symmetrically placed on a horizontally oriented cork ring that exhibited an inside diameter of 3.5 inches (8.9crn.) an outside diameter of 5.5 inches (1 4 cm.) and a thickness of one inch (2.5 cm.). A one pound (0.45 kg.) steel ball, 1.9 inches (4.8 cm.) in diameter, is dropped from a height of 48 inches cm.) above the sample, unless otherwise specified. The dent created by the impact of the ball on the sample was examined and the point of maximum displacement on the lower surface was measured. The distance of this displacement from the plane of the panel will hereinafter be referred to as X.

Cantilever deflector test 2 x 5 inch (5 x 25 cm.) strip was cut from the panel and secured with its major surfaces in a horizontal plane by clamping the strip along a line 1.5 inch (3.8 cm.) from and parallel to one of the ends. A force acting vertically downward was applied along a second line spaced one inch (2.5 cm.) from and parallel to the free end of the strip so as to cause this line to traverse a vertical distance of 2.0 inches (5.0 cm.) in one minute, at the end of which time the force is removed. Following a one minute interval the residual vertical deflection of the strip is measured at a point on the line along which the force was applied.

The polyhexamethylene adipamide yarns or cords employed in the following examples exhibit a tenacity and elongation at break of approximately 9.2 grams per denier and 19% respectively. The rate of stress decay was about 24% of the initial value over a period of 4000 minutes givhen measured using the procedure described hereinbeore.

EXAMPLE I This example illustrates the preparation of a pre-stressed reinforced panel of the present invention.

Using the apparatus described hereinabove, two crosslaid cord groups were strung between pairs of anchor pins. The cord was prepared by twisting together two twisted yarns of high tenacity polyhexamethylene adipamide filaments. The cord exhibits a maximum elongation of 25% at ambient temperature. Each yarn contains 140 filaments of six denier each. During pretensioning the cords were elongated by about of their relaxed length. The cords of one group contacted those of the adajacent group at substantially all of the cross-over points. The lower group was located about 0.09 inches (0.23 cm.) above and parallel to the main plate 3. Adjacent parallel cords were spaced 0.13 inch (0.3 cm.) apart.

A concrete slurry was prepared using 2.8 parts by weight of a commercial Type 3 (high early strength) portland cement, 6.1 parts of sand, and 1.1 part water. The sand was a round-grain silica type supplied by Fisher Scientific Co. These ingredients were thoroughly mixed together and the resultant slurry poured onto the center of the reinforcing network. The bottom plate 6 was vibrated during and following pouring of the concrete to facilitate distribution of the slurry and escape of entrapped air. The viscosity of the concrete slurry was sufiicient to retain it substantially entirely within the desired area. Preferably the concrete is spread over the cross-laid groups using a spatula or similar tool. The top plate 7 was then placed in position and urged downward until it reached the top of the height adjusting screws, which were located about 0.2 inch (0.5 cm.) above the main plate. The top plate was substantially parallel with respect to both the main plate and reinforcing network. The vibrators were then turned off and the cement was allowed to cure for about 24 hours, after which time the cords comprising the cross-laid groups were cut at their point of emergence from the concrete. The minor amount of concrete which had migrated beyond the area of the cross-laid groups was removed using a diamond saw.

The thickness of the test sample was about 0.2 inches (0.5 cm.) and the value of X, as determined by the falling ball impact test, was 0.063 inches (0.16 cm). The sample did not shatter, although some concrete fell out from the area of impact.

EXAMPLE II This example illustrates the preparation of reinforced panels using a high-speed curing system for the concrete. The apparatus was similar to that employed in Example I, with the exception that the bottom plate was replaced by the screen portion of a papermakers handsheet mold with a rigid screen surface. The screen exhibited a length of 9 inches along each side. A vacuum line was connected below the screen. A piece of filter paper was then placed over the screen and two cross-laid warps of the type employed in Example I were arranged about 0.1 inches (0.25 cm.) above the screen and elongated by about 10% of their relaxed length.

The concrete mix was prepared using 18 parts by weight of sharp-grain silica sand and 7.5 parts of Type 3 portland cement. The sand consisted of a mixture graded between 16- and 40-mesh. This dry mixture was distributed throughout and over the cross-laid groups and packed down with the aid of suction supplied through the screen of the handsheet mold. The resulting layer was about 0.2 inches (0.5 cm.) thick. The concrete layer was covered with a sheet of filter paper and the upright portion of a square-shaped handsheet mold was placed in position on top of the filter paper to form a crude dam. The crosslaid groups and concrete mixture prevented contact between the upright and screen portions of the hand-sheet mold. About 2 liters of an aqueous solution of approximately 20% by weight sodium silicate (made from N grade high silicate solution, obtained from the Philadelphia Quartz Company) was poured over substantially the entire surface of the upper sheet of filter paper and was pulled through the dry concrete mix using suction, which was maintained throughout subsequent treating of the sample. Steam was then applied over the silicate solution to heat it and after a minute the upper part of the handsheet mold was lifted so that the remaining solution could run off, and the upper filter paper was removed; the steaming was continued with the upper box section in place for an additional minute. About 2 liters of a 21.5% by weight aqueous solution of calcium chloride was then poured on the sample after which steam was applied for an additional minute. The excess calcium chloride solution was then poured off and steaming was continued for a fourth and final minute. At the end of this time the reinforcing network was severed at the point of emergence from the concrete.

The sample was flexed to the point to which cracks appeared in the concrete. The cracks were not readily apparent when the sample was allowed to return to its original form.

EXAMPLE 111 This example illustrates a concrete panel reinforced with a 2-1ayer reinforcing network comprised of a zerotwist interlaced high tenacity nylon yarn of 1680 denier which contained 280 filaments. The network was arranged on the pretensioning apparatus as described in Example I and the yarns were elongated by about 13% of their relaxed length.

The composition of the concrete mixture Was as follows:

Parts by weight Round Grain Silica Sand 61 Type 3 Portland cement 28 Water 11 The concrete panel was prepared as described in Example I. The falling ball impact test yielded a value for X of 0.095 in. (0.24 cm.) and there was no indication of failure in the adhesion of untwisted yarn to concrete.

EXAMPLE IV This example illustrates a reinforced panel in which cords without stress have been substituted for the pretensioned filamentary members of the present invention.

Employing the apparatus and nylon cord of Example I, a reinforcing network of three groups laid crosswise was prepared. The cords of the uppermost group were positioned to be above the spaces between adjacent cords of the bottom warp. The cords in the center group were 0.125 inches (0.31 cm.) apart and the cords on either side were 0.25 inches (0.63 cm.) apart. The bottom group was about 0.08 inch (0.2 cm.) above the bottom plate of the apparatus. No pretensioning was applied to the cords.

A concrete mix was prepared using the proportions disclosed in Example III.

The mix was distributed and processed as in Example I, with the exception that the mix was cured for 12 hours, after which the panel was removed from the pretensioning apparatus and immersed in Water for about 24 hours. The reinforcing network appeared to be equidistant from either side of the 0.21 inch (0.54 cm.) thick panel.

The falling ball impact test described hereinabove yielded a value for X of 0.37 inches (0.94 cm.).

EXAMPLE V This example illustrates a reinforcing panel employing a reinforcing network in which steel wires have been substituted for the filamentary members of the present invention.

The two cross-laid groups were formed as described in, Example I using steel music wire that exhibited a diameter of 0.012 inches (0.030 cm.) The wire was elongated by about 1% of its relaxed length which is near the breaking limit. The concrete mix was prepared, poured and cured as described in Example III. It had a thickness of 0.20 (0.54 cm.) inch. The falling ball impact test yielded a value for X of 0.61 inches (1.5 cm.). An appreciable amount of concrete broke away from the area of impact. A residual reflection of 1.1 inches 2.8 cm.) was observed following the Cantilever Deflection Test. The elongation at break of the steel wire is less than EXAMPLE VI A prestressed concrete panel was fabricated in which the reinforcing network was located close to one surface of the panel instead of being centrally placed. This was done by arranging the network close to the bottom plate of the pretensioning apparatus such that the warp almost touched the plate. The mortar composition and fabrication procedure are described in Example I. The sample exhibited a thickness of 0.19 inch (0.48 cm.). In the sample thus made the cross-laid groups were located closer to one surface than to the other. The surface close to the fibers will be referred to as the back and the other surface as the front. It was found that the impact resistance of this sample is much higher when the impact is on the front surface than when the impact i on the back surface. The Falling Ball Impact Test was performed on the front face of the sample from a height of 36 inches (91 cm.) and yielded a value for X of 0.072 inch (0.18 cm.). The damage to the sample was confined to the immediate area of impact. An asymmetrically reinforced sample generally exhibits slightly better impact resistance than the symmetrically reinforced samples, provided the impact is on the front surface.

EXAMPLE VII Two commercial cement-asbestos sidings, viz. Rockshake and Plastic Surface, both 0.18 inch thick and obtained from Johns-Manville Corp., were subjected to the Falling Ball Impact Test using 4 x 4 inch (10 x 10 cm.)-samples. All samples shattered badly when the ball was dropped from a height of 48 inches (120 cm.).

EXAMPLE VIII This example illustrates the fabrication of a prestressed panel with a layer of polystyrene foam bonded to one surface.

A square sheet of polystyrene foam exhibiting a length of 12.9 inches (33 cm.) along each side and an average thickness of 0.55 inches (1.4 cm.) was placed on the bottom place of the pretensioning apparatus. A two-layer reinforcing network exhibiting substantially the same dimensions as the foam sheet was formed as described in Example I. The plane defined by the warps was parallel to the upper surface of the foam sheet. The lower group was located about 0.09 inches (0.23 cm.) above the foam sheet. The groups were then elongated by of their relaxed length. A concrete mix was prepared, poured, and cured as described in Example IV to yield a panel having a total thickness of 0.75 inch.

A 4 x 4 inch (10 x 10 cm.)-sample was cut from the panel, placed with the foam side down and subjected to the Falling Ball Impact Test using a ball drop height of 48 inches (120 cm.). The impact created a small dent in the concrete. A value for X could not be determined due to the presence of the foam backing. There appeared to be no cracking or delamination of the concrete from the backing. The cement-foam composite exhibits good mechanical properties together with a high degree of resistance to weather and excellent thermal and acoustical insulating properties.

EXAMPLE IX This example illustrates a prestressed panel containing a concrete-latex matrix.

A three-layer reinforcing network was prepared as described in Example IV and the cords elongated by 11.2% of their relaxed length. A concrete mix was prepared using the following proportions by weight of ingredients.

The concrete mix was poured and set as described for Example I. A 4 x 4 x 0.22 inch (10 x 10 x 0.56 cm.)- sample was subjected to the Falling Ball Impact Test and yielded a value for X of 0.02 inches (0.05 cm.) using a ball drop height of 48 inches cm.).

Molecularly oriented filamentary structures of representative organic polymers chosen from other generic classes that would not be affected by the concete slurry (e.g., polyolefins, acrylic and vinyl polymers, etc.) may be substituted for nylon in pretensioned concrete reinforcing networks, the only provision being that these polymers exhibit the degree of tenacity and elastic elongation required to impart the form recovery and crackclosing ability that characterize the products of the present invention. Other cements such as high alumina cement, Keenes cement etc. can also be employed.

The reinforced concrete panels of the present invention may be used as material for residential siding and roofing and as prefabricated inside wall and ceiling structures.

The specific embodiments and practices herein described have been presented in order to exemplify this invention and should not be regarded as limitations. Various modifications, substitutions and combinations of matrix and reinforcing network may be effected without departing in spirit or scope from the broader aspects of the present invention.

What is claimed is:

1. A thin prestressed reinforced concrete structure under one inch in thickness comprising at least two crosslaid groups made up of yarns of synthetic organic polymeric continuous filaments embedded in a cementitious matrix wherein the yarns have a tenacity of at least 5 grams per denier, an elongation-at-break of at least 9% and a rate of stress decay no greater than 40% of the initial value over a period of 4000 minutes at room temperature, said yarns being elongated at least about 5% and having further recoverable elongation, said groups imparting a compressional stress to the composite structure.

2. The structure of claim 1 wherein the yarns are nylon.

3. The structure of claim 1 wherein portland cement is employed as the cementitious matrix.

References Cited UNITED STATES PATENTS 2,652,093 9/1953 Burton. 2,825,117 3/1958 Evans et al. 2,827,414 3/1958 Bussard et al. 2,836,529 5/ 1958 Morris 52309 3,084,910 4/1963 Allers et al 52223 3,208,838 9/1965 Fischer et al. 51-293 3,262,230 7/1966 Seymour et al. 51-206 3,262,231 7/1966 Polch 51206 FOREIGN PATENTS 603,563 4/1960 Italy.

FRANK L. ABBOTT, Primary Examiner JAMES L. RIDGILL, JR., Assistant Examiner US. Cl. X.-R. 

